Process for multistage conversion of a charge comprising olefins with four, five or more carbon atoms, with the aim of producing propylene

ABSTRACT

The invention relates to a process for production of propylene in particular from a C4 and/or C5 cut from steam cracking and/or catalytic cracking, preferably comprising both butenes and pentenes, said process comprising at least one oligomerization stage, followed by a stage of catalytic cracking of the oligomers formed. 
     Preliminary oligomerization, in particular of a wide fraction of the charge, makes it possible to optimize the yields, the conversion, and the selectivity for propylene, relative to direct cracking. It also makes it possible for cracking to be carried out in a fixed, moving, or fluidized bed, optionally with co-production of oligomers for uses other than the production of propylene.

The invention relates to a process for production of propylene startingfrom light hydrocarbon fractions in particular comprising butenes and/orpentenes.

It relates more particularly to a process by which an olefinic charge,i.e. comprising olefins, hydrocarbons in which the number of carbons isgreater than or equal to 4, for example a C4 and/or C5 fraction (withthe term Cn indicating a hydrocarbon cut with n carbon atoms), forexample from steam cracking or from FCC, can be converted at leastpartially to propylene. The term FCC, the abbreviation of the expressionFluid Catalytic Cracking, denotes fluidized-bed catalytic cracking. Ingeneral, and according to the present invention, the term FCC denotesthe conventional process used in the refinery, of catalytic cracking ofheavy petroleum fractions, using a charge mainly boiling aboveapproximately 350° C. (at least 50 wt. %, generally at least 70 wt. %and often 100 wt. % of the charge boiling above 350° C.), for examplevacuum distillate, or optionally atmospheric residue.

These C4/C5 olefinic cuts are available in large, often surplusquantity, in oil refineries and steam cracking installations. However,their recycling is problematic:

-   -   their recycling to steam cracking presents problems (the yields        of light olefins are lower than with the paraffinic cuts and        they have high coking tendency),    -   their recycling to FCC can scarcely be envisaged as they are        very unreactive in the conditions of FCC, which are adapted to        the vacuum distillate charge. Their recycling to FCC would        therefore require the use of harsher conditions or specific        catalysts, which would alter the operation of FCC.

The charge of the process according to the invention can also comprise asteam cracked gasoline or an FCC gasoline, or some other olefinicgasoline. (By gasoline is generally meant a hydrocarbon cut obtained forthe most part at least from at least one conversion or synthesis unit(such as FCC, visbreaking, coking, Fischer-Tropsch unit, etc.) and wherethe largest proportion and typically at least 90 wt. % of this cut iscomprised of hydrocarbons having at least 5 carbon atoms and a boilingpoint less than or equal to approximately 220° C.).

The olefinic cut constituting the charge of the process is thereforepreferably selected from those defined previously, or comprises amixture of those defined previously. A typical charge often comprisesbutenes and/or pentenes in notable or considerable quantity, but canalso comprise ethylene, optionally small quantities of unfractionatedpropylene, hexenes, olefins having from 7 to 10 carbon atoms, andolefinic gasoline cuts. Most often, the charge is not purely olefinicbut also comprises paraffins (in particular n-butane and/or isobutane,pentanes, and sometimes aromatics, in particular benzene and/or toluene,and/or xylenes. It can comprise isobutene and/or iso-amylenes.

The charge also often comprises highly unsaturated compounds: dienes(diolefins) in particular with 4 or 5 carbon atoms (especiallybutadiene).

The charge is typically a light charge, whose final distillation point(according to the TBP process, well known to a person skilled in theart), or at least the point at which 90 wt. % of the charge isdistilled, is very generally below 320° C., and generally below 250° C.

The process for conversion of a charge comprising C4 and/or C5 olefinichydrocarbons, to a cut comprising propylene, the object of the presentinvention, uses the series of the following stages successively:

-   -   a stage of oligomerization and/or co-oligomerization of the        butenes and/or pentenes contained in the charge, in particular        to obtain higher olefins, in particular with number of carbon        atoms for the most part greater than or equal to eight. If the        charge, according to a variant of the process according to the        invention, comprises ethylene, the reactions of        co-oligomerization can also produce a certain quantity of C6 or        C7 olefins,    -   a stage of catalytic cracking of the higher olefins thus        produced.

Often, compounds formed by the addition of n identical olefins arecalled oligomers of olefins, and compounds formed by the addition of nolefins of which at least two are different are called co-oligomers.

According to the invention, and hereinafter in the present descriptionas well as in the claims, the term oligomers (and the terms oligomerizeand oligomerization) will be used more widely, applying it to higherolefins formed by addition of n identical and/or different olefins (theterm thus also applying to a cut comprising co-oligomers).

Oligomerization differs from polymerization by addition of molecules inlimited number, the aforementioned figure n being, for the most part byweight at least oligomers, comprised between 2 and 10, inclusive, andgenerally between 2 and 5, in particular between 2 and 4. The oligomersmay however comprise traces of olefins that have been oligomerized withn>10. Generally these traces represent less than 5 wt. % relative to theoligomers formed.

The installation for applying the process according to the invention ispreferably installed near or on a refining site (oil refinery), or apetrochemical works (generally steam cracker).

Prior Art:

A known process for production of propylene, apart from the conventionalprocesses for production of FCC and steam cracking (in which propyleneis a co-product with other products such as in particular gasoline orethylene), is the process for metathesis which converts anethylene+n-butene mixture to propylene. This process is described inFrench patent FR 2 668 595.

One of the advantages of the process according to the invention,relative to metathesis, is that it produces propylene from all of theolefinic compounds of the C4 and C5 cuts and optionally of hydrocarboncuts with a larger number of carbon atoms, in particular gasoline, anddoes not require massive consumption of ethylene, which is a high-costproduct. If it is applied at a steam cracking site, the processaccording to the invention not only makes it possible not to useethylene as charge, but also to co-produce ethylene with the propylene.As the co-production of ethylene is typically less than that ofpropylene, this makes it possible to improve the propylene to ethyleneratio of the steam cracker.

Moreover, if it is applied in an oil refinery, the process according tothe invention makes it possible conversely to valorize as necessary (inaddition to C4/C5 cuts) relatively limited and/or difficultlyvalorizable quantities of ethylene, which is often the case in therefinery.

Single-stage processes for production of propylene from C4 and C5olefinic cuts are also known:

-   -   in particular a process is known that consists essentially of        fluidized-bed catalytic cracking, by technology close to        conventional FCC, but operating in conditions of high        temperatures and high severity, in particular: temperature at        riser outlet close to 700° C. (“riser” is the vertical rising        pipe with ascending circulation of catalyst and reaction        charge). A drawback of this type of process, sometimes called        petrochemical FCC, is that it produces overcracking of the        gasoline contained in the charge which thus lowers the yield of        the latter. The paraffins contained in the charge are also        subject to severe temperature conditions, which can cause        thermal cracking and the formation of light compounds which are        poorly valorizable by this process, for example compounds        lighter than propylene. Moreover, the declared propylene yield        barely exceeds 30%, even in conditions of very high severity.    -   Another process for production of propylene is a fluidized-bed        process using a zeolite ZSM-5 as catalyst. This process is        described in international application WO 01/04237 as well as in        the article “Maximizing Refinery Propylene Production Using        ZSM-5 Technology”, which appeared in the journal “Hart's Fuel        Technology and Management”, issue of May 1998. The typical        operating conditions of this process are a temperature in the        region of 600° C., and a pressure from 0.1 to 0.2 MPa. In these        conditions, the propylene yield is approximately 30% and can        rise to 50% with recycling of the unreacted C4 and C5 cuts.

A drawback of this process is that it is quite demanding in terms ofsevere operating conditions, and the definition of the incoming cuts,olefinic C4 and C5, which cannot be gasolines with number of carbonatoms greater than 5, as in the case of the present invention which cantreat a gasoline cut. Another drawback is that the paraffins in thecharge, which pass through the reactor without being convertedcatalytically, can be cracked partially, in particular thermally, withundesirable formation of light compounds.

In the family of single-stage processes, a process described in thearticle “Production of Propylene from Low Valued Olefins”, whichappeared in the journal “Hydrocarbon Engineering” of May 1999 can alsobe mentioned. This is a fixed-bed process in which the catalyst is atype ZSM-5 zeolite acting in the presence of steam. The temperature isclose to 500° C. and the pressure is comprised between 0.1 and 0.2 MPa.The declared cycle time is of the order of 1000 hours. The catalyst isregenerated in situ and its total life, i.e. the length of time it isused in the reactor before it is renewed completely, is approximately 15months. The declared propylene yield is approximately 40% and could riseto 60% with recycling of the unreacted C4 and C5 cuts. This processmakes it possible to obtain a relatively high propylene yield. However,it requires the use of steam, and the catalyst cycle time is not veryhigh.

A process described in international application WO 99/29805 and inpatent EP-A-1 061 116 can also be mentioried. This is a process using atype MFI zeolite catalyst with a high Si/Al ratio (from 180 to 1000) tolimit the hydrogen transfer reactions which are responsible for theproduction of dienes and aromatics. The temperature is close to 550° C.,the pressure is close to 0.1 MPa, and the space velocity is comprisedbetween 10 h⁻¹ and 30 h⁻¹. The propylene yield is comprised between 30and 50%, relative to the quantity of olefins contained in the charge. Itcan therefore be estimated at typically less than 30% referred directlyto the incoming charge.

A process described in patent EP-B-0 109 059 can also be mentioned. Thisis a process using a type ZSM-5 or ZSM-11 zeolite catalyst possessingspecial characteristics, used with a high space velocity. This processalso describes in one variant, the sequence of oligomerization of a C4cut, removal of unreacted butanes, and catalytic cracking of theoligomers. The purpose of the oligomerization is removal of the butanesbefore the cracking stage, via an oligomerization of the butenesfacilitating butanes/oligomers fractionation whereas butanes/butenesfractionation is difficult. The olefinic C5 cuts are cracked directly.There is no mention of technical means or special process arrangementswhen the charge comprises isobutene and/or isopentene, and/orisoamylenes.

Finally, U.S. Pat. No. 6,049,017, which describes a process forproduction of propylene and ethylene from olefinic C4 cuts comprising astage of removal of isobutene by etherification can also be mentioned.In the process according to the present invention, the removal ofisobutene, even if it is possible or preferable, is not indispensable,as will be explained later.

DETAILED DESCRIPTION OF THE INVENTION

In general, relative to single-stage processes for conversion that use asingle catalyst and a single set of operating conditions, the processaccording to the invention, which uses two separate stages, makes itpossible to increase the chain length of the olefins before they aresent to the cracking stage so as to make them more reactive to saidcracking, and to optimize each of the stages from the standpoint of thecatalyst and the operating conditions, as will be discussed in moredetail later.

The different operating conditions make it possible, in particular inthe oligomerization stage, to promote the reactions of addition, inparticular by using a relatively high pressure, whereas it is preferableto use a relatively low pressure and a higher temperature for thecracking stage. Thus, in particular, the propylene formed in thecracking stage has very little tendency to oligomerize once formed.

It has also been found that preliminary oligomerization of a chargecomprising both C4 olefins and at least a notable quantity of otherolefins of the group comprising C2, C5 and C6 olefins, in particular C5and/or C6, would lead to increased yields and to better selectivity forpropylene.

In particular, oligomerization (probably with partialco-oligomerization) of a mixture comprising C4 and C5 olefins, or C4 andC5 and C6, or C4 and C2 and C5, leads to improved propylene yields(after cracking), to greater conversion, and to operating conditionswhich are easier to apply, than if only the C4 cut was oligomerized, theC5 olefins in particular being cracked without preliminaryoligomerization. The advantage of this co-oligomerization is notablewhen the quantity of C5 cut oligomerized is sufficient.

Among the preferred charges of the process according to the invention,which are fed to the oligomerization stage b), charges are foundcomprising at least 50 wt. % and often at least 70 wt. % or even more ofC4+C5+C6 fractions, and which comprise olefins of at least two of thefractions C4, C5, and C6, and in particular a charge:

-   comprising an olefinic C4 cut (i.e. comprising olefins, optionally    with other compounds, for example paraffins), the charge comprising    for example at least 10 wt. % of C4 olefins, and also comprising C5    and/or C6 olefins, for example at least 10 wt. %, with a mass ratio:    R1=C5 Olefins+C6 Olefins/C4 Olefins which is greater than 0.15 and    for example 0.2<R1<5 in particular 0.3<R1<3 and in particular    0.5<R1<2 and more particularly 0.7<R1<1.5.-   or comprising an olefinic C4 cut, the charge comprising for example    at least 10 wt. % of C4 olefins, and also comprising C5 olefins, for    example at least 10 wt. %, with a mass ratio:    R2=C5 Olefins/C4 Olefins greater than 0.15 and for example 0.2<R2<5    in particular 0.3<R2<3, in particular 0.5<R2<2 and more particularly    0.7<R2<1.5.

These charges comprising C4 and C5 olefins can also comprise C6 olefins;they can also be practically free from C6 olefins, with for example amass ratio:R3=C4 Olefins+C5 Olefins/C6 Olefins greater than 10, the C6 olefinsbeing sent for example to the cracking stage, mixed with the oligomers,without being subjected to preliminary oligomerization.

These charges give good propylene yields, after oligomerization andcracking according to the process of the invention. It is thought thatthe C4 and C5 olefins, in particular the fractions of C9 co-dimers,resulting from dimerization of a butene and of a pentene give betterpropylene yields and a higher propylene/ethylene ratio than by directcracking of C4 or C5 olefins, in particular because a notable fractionof C9 dimer can crack giving 3 molecules of propylene.

The fractions resulting from catalytic cracking of the oligomerstypically contain relatively small quantities of olefinic C4 and C5cuts. The majority of the charge is typically an external olefinicfraction, of fresh charge, i.e. not received from the effluents of Staged) of the process according to the invention, for example one or morecharges received from the effluents of a steam cracker (cracking naphthafor example), and/or from an FCC mainly cracking vacuum distillate. Thismeans in particular that it is possible to oligomerize a chargecomprising a sufficient quantity of C5 olefins, complying with thevalues of the ratios R1 and R2 given above.

The process according to the invention leads to several advantages:

-   Greater flexibility in the choice of charges: not only olefinic C4    cuts, but also with C5 and/or C6, or even C7 olefinic fractions, and    optionally ethylene, which can be the feed for oligomerization    and/or relatively heavy olefinic gasoline that can easily be    introduced in the cracking stage.-   A higher propylene yield.-   A higher propylene/ethylene ratio.-   A higher degree of conversion in the cracking stage, owing to the    increased reactivity of the oligomers.-   A longer cycle time of the cracking catalysts (and of the    oligomerization catalysts), it being possible to carry out cracking    in milder conditions, in particular at a lower temperature. This    longer cycle time means that one or more fixed-bed or moving-bed    catalytic cracking reactors can be used without operational    problems, and the use of the more expensive fluidized-bed reactors    can be avoided.

The invention therefore proposes a process for catalytic conversion of ahydrocarbon charge comprising olefins with 4 and/or 5 carbon atoms, saidprocess being characterized by the following succession of stages:

-   at least one Stage b) or b1) or b3) of oligomerization (the Stages    b), b1) and b3) will be explained later), in which a catalytic    oligomerization of the olefins with 4 and/or 5 carbon atoms    contained in the charge, to higher olefins, i.e. to oligomers having    a number of carbon atoms for the most part greater than or equal to    eight, is carried out in at least one reactor, then,-   a Stage d) in which catalytic cracking of at least a proportion,    generally a substantial proportion (such as at least 20 wt. % or at    least 30 wt. %), and often the greater proportion (more than 50 wt.    %, often more than 70%, or even 100%) at least of the oligomers    produced, is carried out in a reactor separate from the    oligomerization reactor, to produce in particular propylene.

Before the charge is introduced into the unit, it will preferably bepossible for it to undergo selective hydrogenation first, in a Stage a)in order to remove the diolefins and other acetylenic impurities whichare often present in the charge. It has in fact been found that thesevarious highly unsaturated compounds contribute to a certaindeactivation of the oligomerization catalyst and that the selectivehydrogenation makes it possible to increase the quantity of olefins thatcan be converted.

According to a variant of the process according to the invention, theeffluent from the oligomerization stage b) is subjected to afractionation stage c) comprising a distillation for separating at leasta proportion of the compounds with 4 and/or 5 carbon atoms, which isoften evacuated directly without feeding the catalytic cracking reactor.The C4/C5 fraction that did not react to oligomerization is in factessentially paraffinic and has very low reactivity to catalyticcracking. Its direct evacuation avoids passage through the catalyticcracking reactor which can give rise to the undesirable production ofgas owing to a certain level of thermal cracking. This C4/C5 charge canbe sent to a steam cracking unit, as this paraffinic charge has provedto be a good charge for steam cracking.

It is also possible to evacuate the C6 fraction and/or the C7 fractionfrom the effluent from oligomerization stage b), and-optionally send itto steam cracking, as well as the C3-fraction compounds with 3 carbonatoms or less of these same effluents from oligomerization.

The effluent from the catalytic cracking stage d) is typically subjectedto a fractionation stage e) most often comprising compression of thegases and one or more distillations for separating the effluents andproducing a propylene-rich C3 cut, or practically pure propylene.

A proportion of the compounds with 4 and/or 5 carbon atoms contained inthe effluents from cracking can advantageously be recycled to the inletof Stage b) or of Stage a).

The particular conditions of the various reaction stages of the processaccording to the invention are described in more detail below, accordingto a variant comprising a selective hydrogenation, an oligomerizationand a catalytic cracking integrated on a single site, the charge usedbeing a light cut of C4 and C5 hydrocarbons mainly containing butenes,pentenes, butanes, pentanes as well as, in certain cases, butadiene andpentadiene in variable quantity.

1) Selective Hydrogenation (Stage a)):

The light cut comes typically from a catalytic cracker and/or a steamcracker. The contents of dienes and acetylenics are high when this cutcomes from a steam cracker; that is why the stage of selectivehydrogenation of the dienes and acetylenics to olefins is almostindispensable in this case. It is also preferable in the majority ofcases, as it reduces the coking of the oligomerization catalyst in Stageb), and increases the cycle time of the oligomerization reactor.However, the scope of the invention would not be exceeded if said stageof selective hydrogenation were not included in the process according tothe invention.

The main aim of this first stage is to convert the diolefins (or dienes)to mono-olefins. In fact, the mono-olefins are the source of theoligomers produced in Stage 2. It is therefore desirable to convert thediolefins to mono-olefins. The second aim of this stage is to remove thetraces of acetylenic hydrocarbons which are always present in these cutsand are undesirable compounds for oligomerization, these compounds alsobeing converted to mono-olefins.

When there is a high proportion of diolefins in the cut, the conversioncan be carried out advantageously in two or three reactors in series forbetter control of the selectivity of hydrogenation. Often the charge tobe treated by recycling is diluted with a certain quantity of theeffluent from this selective hydrogenation.

The residual content of diolefins+acetylenics of the effluent fromselective hydrogenation is typically less than approximately 1000 ppm byweight, preferably less than approximately 100 ppm by weight and verypreferably less than 20 ppm by weight. The residual content ofacetylenics can even be less than 10 ppm, or 5 ppm or even 1 ppm byweight.

The quantity of hydrogen required for all of the reactions carried outin this stage is generally adjusted as a function of the composition ofthe cut so as to have advantageously just a slight excess of hydrogenrelative to the stoichiometric.

Generally, this stage of selective hydrogenation is carried out using acatalyst comprising at least one metal selected from the group formed bynickel, palladium, and platinum, deposited on a support comprisingalumina, silica or silica-alumina. Preferably a catalyst is used whichcomprises at least palladium or a palladium compound fixed on arefractory mineral support, for example on an alumina or asilica-alumina. The content of palladium on the support can be typicallyfrom 0.01 to 5 wt. %, preferably from 0.05 to 1 wt. %. Various forms ofpretreatment known to a person skilled in the art can optionally beapplied to these catalysts to improve their hydrogenation selectivitytowards the mono-olefins.

The operating temperature of selective hydrogenation is generallycomprised between 0 and 200° C., the pressure is typically comprisedbetween 0.1 and 5 MPa, often between 0.5 and 5 MPa, the space velocityis typically between 0.5 and 20 m³ per hour per m³ of catalyst, oftenbetween 0.5 and 5 m³ per hour per m³ of catalyst, and the molar ratioH2/(acetylenic+diolefinic compounds) is generally comprised between 0.5and 5 and preferably between 1 and 3.

When a gasoline cut is also used as the feed for catalytic cracking,this cut can also be subjected beforehand to selective hydrogenation,jointly with or separate from that of the C4 and/or C5 cut. When thisselective hydrogenation is carried out jointly, the gasoline canoptionally be separated from the C4 and/or C5 cut upstream of theoligomerization.

Selective hydrogenation is generally carried out using a fixed-bedreactor, with descending co-current flow of the charge to be treated andof the hydrogen, or with descending flow for the charge to be treatedand ascending flow for the hydrogen.

The process of the invention can also comprise one or more optionalstages of purification of the charge (separate from or jointly with theselective hydrogenation) upstream of the oligomerization, which may beuseful or necessary for at least one of the following stages:oligomerization and cracking. The usefulness of these optional stages ofpurification is directly dependent on the catalyst or catalysts used aswell as on the operating conditions and will be obvious to a personskilled in the art for each particular case considered. Thus, the scopeof the invention would not be exceeded if, upstream of theoligomerization, one or more stages of desulphuration, and/or drying,and/or denitrogenation, and/or deoxygenation, were carried out to removeone or more of the following impurities: sulphur, water, nitrogen,oxygen, below 100 ppm, or 10 ppm, or even 1 ppm by weight, in accordancewith conventional techniques.

2) Oligomerization (Stage b)):

The aim of the second stage is to oligomerize the linear, and optionallybranched, C4 and C5 olefins, as well as any other olefins present, forexample and non-limitatively C2 olefins (ethylene) and/or C6 olefins(hexenes), resulting from the preceding stage, to obtain a mixture ofhydrocarbons containing mono-olefins with a number of carbon atoms forthe most part greater than or equal to eight. Typically, starting from aC4 charge, oligomers are obtained in which the number of carbon atoms isto a large extent at least less than or equal to 30, and for the mostpart between 8 and 20.

Oligomerization can be carried out in one or more stages, with one ormore reactors and one or more catalysts. The following description ofthe catalyst and of the operating conditions can apply to any one of thestages and/or to any one of the reactors.

The oligomerization stage can use a catalyst comprising a Lewis acid,for example aluminium chloride, a chloroalkylaluminium, tintetrachloride, boron trifluoride, said Lewis acid often being combinedwith traces of hydrochloric acid, water, tert-butyl chloride, or organicacids.

The selectivities for dimer and for trimer depend of the catalyst and onthe operating conditions. In the present invention, the process foroligomerization is such that a notable or if necessary thoroughconversion of all of the starting olefins is sought.

The catalyst used for the oligomerization stage can also comprisesupported sulphuric acid or supported phosphoric acid, for example onsilica, alumina, or silica-alumina.

The catalyst used for the oligomerization stage can also comprise asulphonic resin (as a non-limiting example, an AMBERLIST resin marketedby the company ROHM & HAAS).

The catalyst used for the oligomerization stage can also comprise asilica-alumina, or preferably an acid solid exhibiting shapeselectivity.

For example, said catalyst can comprise at least one zeolite exhibitingshape selectivity, said zeolite comprising silicon and at least oneelement chosen from the group comprising aluminium, iron, gallium,phosphorus, boron, and preferably aluminium. Said zeolite can forexample be of one of the following structural types: MEL (for exampleZSM-11), MFI (for example ZSM-5), NES, EUO, FER, CHA (for exampleSAPO-34), MFS, MWW, or can also be one of the following zeolites: NU-85,NU-86, NU-88 and IM-5, which also exhibit shape selectivity.

The advantage of these zeolites which exhibit shape selectivity is thatit limits the formation of highly branched oligomers, for exampletri-branched isomers, cracking of which leads to a lowerpropylene/isobutene selectivity, i.e. to a lower propylene isobutenemass ratio.

It is also possible to use several zeolites exhibiting shapeselectivity, for example a type MFI zeolite (for example ZSM-5) combinedwith another zeolite, previously mentioned or of one of the typespreviously mentioned.

The zeolite used can also be mixed with a zeolite that does not exhibitshape selectivity, for example a zeolite Y of structural type FAU.

The zeolite or zeolites can be dispersed in a matrix based on silica,alumina or silica-alumina, the proportion of zeolite (and generally ofzeolite exhibiting shape selectivity) often being comprised between 3and 80 wt. %, in particular between 6 and 50 wt. % and preferablybetween 10 and 45 wt. %.

The zeolite used (or the zeolites used) exhibiting shape selectivitygenerally have an Si/Al ratio greater than 12, preferably greater than40, more preferably greater than 50, and even more preferably greaterthan 80.

The Si/Al ratio can for example be comprised between 40 and 1000. Thismakes it possible to reduce the acidity of the catalyst and thereactions of hydrogen transfer which lead to the formation of paraffinshaving little or no reactivity in the subsequent cracking stage. Thesehigh Si/Al ratios can be obtained at the time of manufacture of thezeolite, or by subsequent dealumination.

The oligomerization catalyst can finally be different from theaforementioned catalysts, if it possesses notable activity inoligomerization.

The catalyst can be used in the solid state, in powder form, or in theform of spheres or extrudates with diameter generally comprised between0.4 and 6 mm, and preferably between 0.6 and 4 mm.

The catalyst can also be used in the form of a suspension in a saturatedhydrocarbon such as hexane or isobutane, or in a halogenated hydrocarbonsuch as methyl chloride. The suspension can be used in a bubbling bed,in particular with particles with average diameter comprised between0.25 and 1 mm and preferably between 0.3 and 0.8 mm, or in finesuspension, with particles of average diameter between 0.02 and 0.25 mmand preferably comprised between 0.03 and 0.20 mm. It is also possibleto use a suspension where the particles are in the colloidal state.

The preferred form of application for the oligomerization reactor isfixed-bed.

The operating conditions are chosen as a function of the catalyst, insuch a way that the reaction takes place at a sufficient rate. Thetemperature (at reactor outlet) can be for example comprised between−100° C. and 350° C., preferably between 0° C. and 310° C., and verypreferably between 70° C. and 310° C., for example between 120° C. and250° C., in particular between 150 and 220° C. Often the temperature ofthe oligomerization stage b) is at least 40° C. lower, preferably atleast 80° C. lower, and very preferably at least 120° C. lower than thatof the catalytic cracking stage d).

The pressure is typically comprised between 0.1 and 10 MPa, andpreferably comprised between 0.1 and 5 MPa, and very preferablycomprised between 0.8 and 4 MPa, and in particular comprised between 1.5and 3.5 MPa. Often the pressure (at reactor outlet) of theoligomerization stage b) is at least 0.5 MPa higher, preferably at least1 MPa higher, and very preferably at least 1.5 MPa higher than that ofthe catalytic cracking stage d).

The SV is generally comprised between 0.1 and 5 m³ per hour per m³ ofcatalyst, and preferably between 0.5 and 4 m³ per hour per m³ ofcatalyst.

The operating conditions are often also optimized as' a function of thecharacteristics of the charge.

It is also possible to use, for the selective hydrogenation stage a) andfor the oligomerization stage b), conditions which are similar, and inparticular pressures which are similar, such as pressures that onlydiffer from one another by 0.5 MPa at most, or even 0.3 MPa at most.This makes it possible for the two reactions to follow one another,optionally without intermediate fractionation or pressurization ordepressurization, or optionally even without intermediate cooling oreven without intermediate heating. It is also possible to carry out thereactions of selective hydrogenation and of oligomerization in twosuccessive beds of the same reactor. The conversion of the C4 and C5olefins during oligomerization generally reaches 70%, or 90% or more,and can even be practically total.

It may be useful in this stage, in the particular conditions discussedbelow, to add a small quantity of ethylene to the charge as thispromotes the formation of oligomers with six or seven carbon atoms (byaddition with the C4/C5 olefins of the charge) and their subsequentcracking to propylene. This makes it possible to valorize the relativelylimited quantities of ethylene available in an oil refinery (saidethylene essentially being produced in FCC). Another situation wherethis arrangement is useful is that of an ethylene supply from a steamcracker, during economic conditions when there is low demand forethylene but the demand for propylene remains high. The quantity ofethylene can then be adjusted to the available surplus. (For comparison,such adjustment is not possible in the process with metathesis, where asmany moles of ethylene are used as of butene). The quantity of ethylenethat can be used is for example comprised between 0.5 and 15 wt. % ofthe oligomerization charge. Typically, the charge of the oligomerizationreactor comprises 0.5 to 15 wt. % of ethylene relative to the sum of theC4, C5 and C6 olefins.

The use of oligomerization at relatively high pressure and lowtemperature relative to that of catalytic cracking makes it possible tooptimize the two types of chemical reactions separately, and to usespecific catalysts. It also makes it possible to increase the cycle timeand life of the oligomerization catalyst, which is subject to far lesssevere conditions, in particular with respect to coking.

Generally, the oligomerization reactor is a fixed bed, uses a catalystcomprising a silica-alumina or preferably at least one zeolite, and verypreferably a zeolite exhibiting shape selectivity (for example a typeMFI zeolite), and operates at a temperature comprised between 70° C. and+310° C., a pressure typically comprised between 0.1 and 5 MPa, and aspace velocity comprised between 0.1 and 5 m³ per hour per m³ ofcatalyst.

According to a variant of the process according to the invention, whichcan be used in particular when the charge contains isobutene, especiallyin substantial or high quantity, the oligomerization stage b) can becarried out in 3 stages:

-   A Stage b1) of limited oligomerizatiori, making it possible to carry    out preferential oligomerization of the more reactive branched    olefins, in particular of isobutene, the linear olefins being in    particular less oligomerized,-   A Stage b2) of fractionation of the effluents from Stage b1), for    example by distillation or any other known fractionation, making it    possible to extract at least one cut comprising di-isobutene and/or,    optionally, tri-isobutene: C8 cut rich in di-isobutene, or    practically pure di-isobutene, or optionally a C8+ cut (C8 and    heavier, optionally also comprising tri-isobutene at the same time    as di-isobutene), said extracted cut being evacuated directly (i.e.    not being fed to the subsequent Stages b3) of oligomerization and d)    of cracking).-   A Stage b3) of final oligomerization of the effluent from Stage b2),    or at least of olefinic C4 and/or C5 fractions, after evacuation of    the aforementioned cut comprising di-isobutene and/or optionally    tri-isobutene.

These stages interact with one another as well as with cracking staged): Stages b1) and b2) make it possible to remove at least partly theisobutene via a product: di-isobutene and/or tri-isobutene for which thecatalytic cracking propylene yields are relatively low, and obtainless-branched oligomers in Stage b3), giving better cracking yields inStage d). The at least partial removal of isobutene before Stage b3)also makes it possible to reduce gum formation in said Stage b3) wheredeep oligomerization of the linear C4 and/or C5 olefins is required.

The variant of the process described above (with limited oligomerizationb1) then final oligomerization b3) after fractionation b2) and at leastpartial removal of the oligomers formed in b1)) can also be applied to acharge comprising isoamylenes (branched C5 olefins) instead ofisobutene, or a charge comprising isobutene and isoamylenes. Thesebranched olefins can be oligomerized much more easily and preferentiallyto their linear homologues, which makes it possible to remove them atleast partially after Stage b1).

Stage b1), which does not aim at the formation of linear olefins whichare good precursors of propylene, can be implemented with a catalystamong those mentioned previously, but also with a zeolite catalysthaving a percentage of zeolite exhibiting shape selectivity lower thanthat of Stage b3), or even with a non-zeolite catalyst, essentiallycomprising an amorphous silica-alumina of medium acidity.

Stage b1) of oligomerization can also use different, very selectiveoperating conditions as it provides very preferential or exclusiveoligomerization of the isobutene (and/or of the isoamylenes) relative tothe n-butenes (linear butenes) and/or the n-pentenes. For example, itwill be possible to use milder conditions in the first stage ofoligomerization relative to the final stage, in particular by using atemperature at least 40° C. lower in the first stage. It is for examplepossible to carry out a first oligomerization b1) with a temperaturecomprised between 20 and 80° C., and a second oligomerization b3) with atemperature above 100° C., or even 120° C. or more. It is possible touse the same catalyst for 3), for example based on silica-alumina, oraltematively different catalysts.

Di-isobutene and tri-isobutene are in fact, for each of these compounds,a mixture of isomers, well known to a person skilled in the art; inparticular there are two isomers for di-isobutene, including2,4,4-trimethyl-2-pentene, with normal boiling point of 104.9° C., whichboils in the gasoline range and has a good octane number. Tri-isobutenecomprises oligomers some of which have a normal boiling point comprisedbetween 196 and 210° C., which can be incorporated at least partly in agasoline base or a kerosene, or a gasoil, depending on the valorizationsrequired. It can also be valorized for uses in the chemical industry.

An extracted cut rich in di-isobutene can be valorized at a high levelas gasoline base, or for other uses, for example in the chemicalindustry etc.

Stage b1) can in particular be applied on a C4 cut alone; a C5, or C2and C5 cut can then be added if necessary to the butenes not convertedin b1) for final oligomerization in Stage b3). It is also possible tocarry out Stage b1) with a charge comprising hydrocarbons other than aC4 cut, for example an olefinic cut with C4 and C5, or C4 and C5 and C6,or C4 and C2, or C4 and C2 and C5, or C4 and C2 and C5 and C6.

When oligomerization is applied in a single Stage b), it is alsopossible, in the same way as after Stage b1), for a proportion of theoligomers produced for example a fraction comprising di-isobutene and/ortri-isobutene, to be removed and evacuated directly.

In all these variants, it will also be possible to evacuate unreactiveC4 and/or C5 fractions after a fractionation stage b2) or c), so as notto obstruct the subsequent stages.

3) Catalytic Cracking (Stage d)):

The charge fed in Stage d) of catalytic cracking typically contains from20 to 100 wt. % of olefins with at least 8 carbon atoms that wereproduced by oligomerization of light olefins with 4 and/or 5 carbonatoms, often from 30 to 100 wt. %, and most often from 50 to 100 wt. %,in particular from 60 to 100 wt. %.

The charge can also comprise other oligomers formed essentially from thegroup comprising C2 to C10 olefins, the total quantity of oligomers withat least 6 carbon atoms being typically from 25 to 100 wt. %, often from35 to 100 wt. %, most often from 55 to 100 wt. %, and in particular from65 to 100 wt. % relative to the charge of Stage d).

The C6 oligomers, formed in particular by addition of ethylene to abutene, or the heavier oligomers formed at least partly from C6 andhigher olefins (C6+) are in fact also good propylene precursors, whichit is also advantageous to use as feed for catalytic cracking.

The catalyst used for the catalytic cracking stage can comprise asilica-alumina. Preferably, however, an acid solid exhibiting shapeselectivity is used.

For example, this catalyst can comprise at least one zeolite exhibitingshape selectivity, said zeolite comprising silicon and at least oneelement selected from the group formed by aluminium, iron, gallium,phosphorus, boron, and preferably aluminium. Said zeolite exhibitingshape selectivity can be of one of the following structural types: MEL(for example ZSM-11), MFI (for example ZSM-5), NES, EUO, FER, CHA (forexample SAPO-34), MFS, MWW, or can also be one of the followingzeolites: NU-85, NU-86, NU-88 and IM-5, which also exhibit shapeselectivity.

The advantage of these zeolites exhibiting shape selectivity is that itleads to better propylene/isobutene selectivity (higherpropylene/isobutene ratio in the effluents from cracking).

It is also possible to use several zeolites exhibiting shapeselectivity, for example a zeolite of the MFI type (for example ZSM-5)combined with another zeolite exhibiting shape selectivity, mentionedabove or of one of the types mentioned above.

The zeolite or zeolites exhibiting shape selectivity, from the groupcomprising the zeolites of one of the following structural types: MEL(for example ZSM-11), MFI (for example ZSM-5), NES, EUO, FER, CHA (forexample SAPO-34), MFS, MWW, or from the group of the following zeolites:NU-85, NU-86, NU-88 and IM-5, can also be mixed with a zeolite that doesnot exhibit shape selectivity, such as a zeolite Y of structural typeFAU.

Often a catalyst is used which comprises one or more zeolites exhibitingshape selectivity, the proportion of zeolite(s) exhibiting shapeselectivity being comprised between 70 and 100 wt. %, inclusive,relative to the total quantity of zeolite(s). In particular a catalystcan be used for which the proportion of zeolite(s) exhibiting shapeselectivity is comprised between 80 and 100 wt. % relative to the totalquantity of zeolite(s), and even a catalyst in which the zeolite orzeolites all exhibit shape selectivity.

The zeolite or zeolites can be dispersed in a matrix based on silica,alumina or silica-alumina, the proportion of zeolite (and generally ofzeolite exhibiting shape selectivity) often being comprised between 3and 80 wt. %, preferably between 8 and 70 wt. %, for example between 15and 60 wt. %, in particular between 20 and 50 wt. %. The zeolite (orzeolites) used, exhibiting shape selectivity, generally has (have) anSi/Al ratio greater than 12, preferably greater than 20, more preferablygreater than 50, and often greater than 80. It can for example becomprised between 40 and 500. This makes it possible in particular toreduce the acidity of the catalyst and the reactions of hydrogentransfer which lead to the formation of paraffins at the expense ofpropylene formation.

Such high Si/Al ratios can be obtained at the time of manufacture of thezeolite, or by subsequent dealumination.

Finally the catalytic cracking catalyst can be different from theaforementioned catalysts, if it possesses a notable activity incatalytic cracking for the production of propylene.

The aforementioned Si/Al ratios may be different for the oligomerizationand cracking catalysts, which permits their respective optimization.

The catalyst can be used in the solid state, as powder if using afluidized-bed reactor, for example with an average particle sizecomprised between 0.02 and 0.5 mm, preferably between 0.04 and 0.15 mm.Typically, the catalyst then circulates continuously from the crackingreactor to a regeneration zone, then returns to the reactor. Thetechnology used is then similar or identical to that of the FCC process.

Oligomers can also be cracked according to the FCC process, mixed withheavy gasoil and/or vacuum distillate (or in a separate riser). Thisvariant is the object of a separate patent application, simultaneouswith the present application. In such a case, the quantity of oligomersadded to the heavy charge (heavy gasoil and/or vacuum distillate) is inmost cases relatively low: typically between 3 and 40 wt. %, inparticular from 4 to 30%, or from 4 to 26 wt. % of the total charge.

The charge treated is typically an olefinic cut (C4 and/or C5 and/or C6and/or C2) obtained from FCC. In general, only a limited quantity ofolefinic fraction is treated, which leads after cracking of theoligomers in the FCC (with the vacuum distillate and/or the heavygasoil) to an increase in the quantity of gas compatible with thecracked gas compressor and the gas treatment installation and lightC5/C6 fractions. The feed for oligomerization (after any purificationtreatments) can then be a predetermined quantity of olefinic cut (C4and/or C5 and/or C6 and/or C2), the complement relative to theproduction of the FCC being evacuated and fractionated so as to producea purge flow and prevent excessive swelling of the loop of C5 lightproducts around the FCC and of oligomerization, in particular anaccumulation of isobutene and/or isoamylenes. These compounds in facthave a tendency to oligomerize rapidly but to crack again (during thecracking stage) in notable quantity or completely back to the startingproduct, producing only very little propylene. A purge can prevent anincreasing accumulation of isobutene and/or isoamylenes.

Alternatively, it can be used in a fixed bed or in a moving bed, in theform of spheres or extrudates with diameter generally comprised between0.4 and 6 mm, preferably between 0.6 and 4 mm.

According to one of the preferred embodiments of the process accordingto the invention, a moving-bed of catalyst, for example of spheres withdiameter from 1 to 3 mm, is used for the cracking stage d). The catalystthen circulates continuously or semi-continuously from the crackingreactor to a regeneration zone, then returns to the reactor.

According to another preferred embodiment, at least 2 fixed-bed reactorswith cyclic operation are used, one reactor being in operation (crackingphase) and another reactor in the regeneration phase, according to the“swing” reactor technique, using the term that is well known to a personskilled in the art. When the regeneration of the second reactor isfinished, the charge is swung to the second reactor, and the catalyst ofthe first reactor is regenerated. It is also possible to use threereactors, with two reactors in operation and one in regeneration, orthree reactors in operation and one in regeneration, or N reactors inoperation and P reactors in regeneration, variants which are consideredaccording to the invention as technical equivalents to swing reactors.

The regeneration phase typically comprises a phase of combustion of thecarbon deposits formed on the catalyst, for example by means of anair/nitrogen mixture or of air with lower oxygen content (for example byrecirculation of fumes), or of air, and can optionally comprise otherphases of treatment and of catalyst regeneration.

Catalytic cracking is usually carried out at a temperature ofapproximately 450 to approximately 650° C. and preferably between 480°C. and 600° C. with a residence time in the reactor of less than 1minute, often from approximately 0.1 to approximately 50 seconds andpreferably from 0.4 to 15 seconds. The operating pressure is generallycomprised between 0.1 and 5 MPa, most often between 0.1 and 1.5 MPa, andpreferably between 0.1 and 0.5 MPa.

The conditions for regeneration of the cracking catalyst generally use atemperature comprised between 300 and 900° C., in particular between 500and 750° C., the pressure most often being close to the crackingpressure, or alternatively close to atmospheric pressure.

According to another embodiment of the process according to theinvention, it is also possible to use the same circulating catalyst foroligomerization and cracking.

This circulation of the catalyst can be implemented in a moving bed orin a fluidized bed. The catalyst then circulates advantageously betweenthree zones: the oligomerization zone, the catalytic cracking zone, anda third zone of catalyst regeneration, this last-mentioned zone inparticular carrying out the removal of the coke deposited on thecatalyst (by controlled combustion by one of the techniques known to aperson skilled in the art). For example, the flow of regeneratedcatalyst leaving the regeneration zone can be divided into two flows,with the first feeding the oligomerization zone, and the second thecracking zone. The catalyst leaving the two reaction zones (after thereaction effluents have first been separated) can then be regenerated inthe common regeneration zone.

The flow of catalyst can also feed the two reaction zones successively(oligomerization then cracking, or vice versa).

The flows of catalyst can if necessary undergo cooling, in particularthe flow feeding the oligomerization zone, or a different cooling, toobtain different operating temperatures in the oligomerization zone andin the cracking zone. This cooling can be obtained for example bybringing the catalyst into contact with a colder gas, or in a heatexchanger. The reaction charges can also be fed at differenttemperatures to achieve this result. It is also possible to operate thecracking reactor with the relatively hotter, regenerated catalyst.

The main variants of implementation of the process according to theinvention are as follows:

-   Variant A: different catalysts for oligomerization and cracking;    fixed-bed oligomerization, preferably with cyclic regeneration of    the catalyst in the same reactor at spaced time intervals, or with    swing reactors; fixed-bed catalytic cracking, preferably with    relatively frequent cyclic regeneration with other swing reactor(s).-   Variant B: different catalysts for oligomerization and cracking;    fixed-bed oligomerization, preferably with cyclic regeneration of    the catalyst in the same reactor at spaced time intervals, or with    swing reactors; moving-bed catalytic cracking (with continuous or    semi-continuous circulation of the catalyst to a regeneration zone).-   Variant C: different catalysts for oligomerization and cracking;    fixed-bed oligomerization, preferably with cyclic regeneration of    the catalyst in the same reactor at spaced time intervals, or with    swing reactors; fluidized-bed catalytic cracking (with continuous    circulation of the catalyst to a regeneration zone).-   Variant D: common catalyst for oligomerization and cracking;    moving-bed oligomerization and cracking (with the common catalyst    circulating continuously or semi-continuously between an    oligomerization zone, a cracking zone, and a regeneration zone).-   Variant E: common catalyst for oligomerization and cracking;    fluidized-bed oligomerization and cracking (with the common catalyst    circulating continuously or semi-continuously between an    oligomerization zone, a cracking zone, and a regeneration zone).

Variants A, B and C can also be implemented with a catalyst suspended ina liquid for the oligomerization stage.

The preferred variants according to the invention are variants A, B andC, in particular with a fixed bed for oligomerization, and the mostpreferred variants are variants A and B.

Generally, the propylene yield relative to the quantity of olefinscontained in the fresh charge of the process is comprised between 30 and60 wt. %, and often between 40 and 60 wt. %.

The invention will be explained in more detail by means of thedescription of FIGS. 1 and 2.

FIG. 1 shows an installation for implementing the process according tothe invention in a first variant with considerable integration betweenthe stages of the process (in particular by recycling).

A C4/C5 charge obtained from a steam cracking unit (not shown in thefigure) is introduced through line 1. Line 1 bis carries hydrogen or ahydrogen-rich gas which is used for the stage of selectivehydrogenation, carried out in a fixed hed in reactor R1 (which cancomprise 2 or 3 reaction zones in series with intermediate cooling ifnecessary). The charge and the hydrogen-rich gas are introduced into thehydrogenation reactor R1 via line 2. R1 is also fed with a recyclingstream circulating in line 13. Reactor R1 is thus fed by two separatelines 2 and 13 in FIG. 1. It is also possible to feed the charges as amixture through a common line. Moreover, the hydrogen can also be fedinside the reactor and not upstream of it. Such variant embodiments orequivalent technical means, which are obvious to a person skilled in theart, also apply to other reactors or separation zones shown in FIGS. 1and 2.

The effluents from reactor R1 feed, via line 3, a fractionation zone S1comprising a stabilization column.

The isobutene can if necessary be extracted at S1 (according to one ofthe techniques disclosed below or any other known techniques), to reducethe quantity or avoid the presence of isobutene in the oligomerizationreactor. Isobutene in fact tends to dimerize to di-isobutene, crackingof which gives relatively low propylene yields, and leads to notablerecracking to isobutene, which therefore tends to accumulate.

It is also possible to extract at least one fraction of the C8oligomers, to reduce the quantity or largely suppress the fraction richin di-isobutene which is subjected to cracking, either followingoligomerization, or after a limited oligomerization, as was explainedabove.

The light products, mainly hydrogen and methane, are evacuated via line4. The selectively hydrogenated C4 cut is introduced via line 5 intooligomerization reactor R2. A recycled olefinic cut, obtained from theeffluents from the catalytic cracking stage, is optionally introducedvia line 10 into the oligomerization reactor. Preferably, this cut canbe sent to the selective hydrogenation stage via the aforementioned line13, rather than to oligomerization.

The effluents from oligomerization are extracted via line 6 andintroduced into a separation zone S2. Zone S2 typically comprises a setof simple and/or reactive and/or extractive distillation columns, notshown in FIG. 1: After distillation of the oligomerization effluents torecover the heavier oligomers, the residual C4/C5 cut, comprising aminority of unconverted olefinic compounds and especially paraffiniccompounds, is evacuated via line 7 a. The oligomers are transferred atleast partly via line 8, and introduced into the catalytic crackingreactor R3.

Another proportion of these oligomers, having thus been separated fromthe oligomerization effluents (or from at least one oligomerizationstage if there are several) by withdrawal or by one or moredistillations, can be evacuated via line 7 c (and is therefore removedfrom the fraction subjected to cracking). This makes it possible toreserve a proportion of these oligomers for uses other than theproduction of propylene, optionally with higher valorization. Propyleneproduction is then less, but the size of the cracking reactor is alsoreduced. As an example, a proportion of the C10 to C14 oligomers can beused as bases for the manufacture of linear or non-linear alkylbenzenes,or as bases for other chemical or petrochemical applications. It is alsopossible to separate oligomerization effluents and evacuate one or morefractions boiling in the distillation range of gasoline, kerosene orgasoil, or domestic heating oil, which can be used as base(s) for themanufacture of these products. This evacuation of a proportion of theoligomers, which are not fed to catalytic cracking, is a notableadvantage of the process according to the invention relative to thesingle-stage processes for conversion of light olefins to propylene,which cannot provide co-production of the oligomers. It can alsocontribute to the indirect elimination of isobutene, when this compoundis present in the oligomerization charge, by separation, for example bydistillation, and evacuation of a fraction of the oligomers comprisingdi-isobutene and/or tri-isobutene (comprising dimers or trimers ofisobutene), for example a C8 or C8+ fraction. The fraction evacuated canbe separated by fractionation of the effluents from oligomerization, forexample by distillation. According to the invention, fractionation isconsidered in the broad sense and also covers a withdrawal of aproportion of the oligomerization effluents or of the oligomers.

An oligomer fraction and/or C4 and/or C5 cut contained in theoligomerization effluents can optionally be recycled to oligomerizationreactor R2 via line 7 b, said rather unreactive fraction making itpossible to reduce the temperature rise in the exothermic reactor R2 (orthe reactors in series if the oligomerization comprises severalreactors). When oligomerization is carried out in two Stages b1) andb3), specific recycling can be carried out for each of the stages(recycling of a proportion of the effluent from the same stage).

A recycling of effluents can also be used in the stage (or stages orreactor or reactors) of selective hydrogenation, and the effluents whichare recycled can be effluents from selective hydrogenation or fromoligomerization, optionally with total recycling around the selectivehydrogenation+oligomerization combination.

The charge of oligomers circulating in line 8 is cracked in catalyticcracking reactor R3. Reactor R3, preferably a fixed-bed or moving-bedreactor, can optionally also be fed with a gasoline fraction introducedvia line 9, so as to increase the quantity of olefins cracked at leastpartially to propylene. The C5 fraction of gasoline, unreactive incracking, and optionally the C₆ fraction, is preferably fed with theC4/C5 charge, to be oligomerized rather than with the main proportion ofthe gasoline fed by line 9.

The total charge of the catalytic cracking unit therefore comprisesoligomers of C4 and/or C5 olefins, and optionally gasoline and/or aquantity (generally relatively small) of ethylene. This total charge isrelatively light, with at least 50 wt. % and generally 80 wt. % and eventypically at least 90 wt. % (and often all) of this charge boiling below250° C. This charge typically does not contain petroleum fractions suchas vacuum distillate, and is therefore very different from the chargesof the FCC units of oil refineries.

The effluents from catalytic cracking unit R3 are evacuated via line 11and are introduced into a separation zone S3, which typically comprisesa gas compressor and distillation means.

When the installation for implementing the invention is on a steamcracking site, it is very useful to be able to use the fractionationtrain of the steam cracker for fractionation of the products. Zone S3 inFIG. 1 therefore represents a common fractionation zone, on the one handof the products circulating in line 11, which come from the catalyticcracking stage (of the process according to the invention), and on theother hand of steam cracking effluents fed via line 16. The effluentsfrom catalytic cracking represent a minor fraction (less than 50 molar%, and often less than 30 molar % of the steam cracking effluents. As avariant, the effluents from catalytic cracking can be mixed with thosefrom steam cracking not upstream of the common separation zone, butafter the effluents from steam cracking have undergone primaryfractionation to remove the gasoline (or at least the heavy gasoline) oreven after the first stage of compression of the gaseous effluents fromsteam cracking.

The invention therefore also proposes a process for conversion combininga stage of steam cracking and a stage of catalytic cracking, preferablywith crossed recycling, in which:

-   a main charge of hydrocarbons representing at least 50 wt. %, and    generally at least 60 wt. % of the total charge is subjected to a    stage of steam cracking,-   a secondary charge of hydrocarbons representing at least 5 wt. %,    and generally at least 10 wt. %, for example between 10 and 40 wt. %    of the total charge (made up of the main charge and the secondary    charge) is subjected to a stage of catalytic cracking d) as    described previously, preferably in a fixed bed or a moving bed,-   the effluents from steam cracking and from catalytic cracking are    cooled,-   the cooled effluents from steam cracking and from catalytic cracking    are fractionated at least partly in a common fractionation zone    (optionally after a separate preliminary fractionation for removing    liquid fractions, most or practically all of the gaseous compounds    from the effluents, in particular those comprising hydrogen and    hydrocarbons comprising 3 carbon atoms or less, preferably being    fractionated in a common fractionation zone), to produce at least    ethylene, propylene, and at least one olefinic cut comprising C4    and/or C5, and preferably C4 and C5, olefins,-   said olefinic cut is subjected to at least one stage of    oligomerization b) as described previously, to produce oligomers,-   at least a proportion of the oligomers is sent to Stage d),-   the effluents from Stage b) are preferably fractionated, in a Stage    c), into at least one fraction rich in oligomers (at least 50 wt. %    o and often at least 70 wt. %, or even 90 wt. %), and at least one    light fraction comprising mainly (at least 50 wt. % and often at    least 70 wt. %, or even 80 wt. %) light hydrocarbons having 5 carbon    atoms or less, for example a fraction which is relatively poor in    olefins mainly comprising C4 and/or C5 paraffins,-   a proportion at least of the light fraction from the preceding stage    is preferably sent to the steam cracking stage.

The last two stages mentioned above are optional but preferred accordingto the invention: The formation of oligomers makes it possible for lightfractions with less than 8 carbon atoms (C7−), or for example with lessthan 6 carbon atoms (C5−), for example a C4/C5 cut, to be separatedeasily by distillation. As these cuts have a low content of olefins, forexample less than 20 wt. %, as a result of oligomerization of most ofthe olefins to heavier compounds, they constitute good steam crackingcharges.

The process for conversion thus defined can also use one or more of thevariants of the process according to the invention mentioned above, andfor example:

-   use of a selective hydrogenation stage a) upstream of the    oligomerization stage b),-   use of extraction of isobutene prior to Stage a) or prior to Stage    b), and after Stage a),-   use of oligomerization in at least two stages as described    previously (Stages b1), b2), b3)),-   evacuation, in Stage b2) or c), of a proportion of the oligomers    (proportion not sent to Stage d)), in particular of a proportion    comprising di-isobutene and/or tri-isobutene-   evacuation (without recycling) of a proportion of the C4 cut of the    effluents from cracking, to carry out an isobutene purge.

The extraction of isobutene from recycled cut(s) can be carried outseparately, or at the same time as removal of isobutene from a fresh(external) charge, for example from a cut received from FCC, in a singleinstallation for treating the recycled cuts and the fresh charge as amixture.

Extraction of the-isobutene can be carried out by extractivedistillation, for example with a solvent which can beN-methylpyrrolidone (NMP) or dimethylsulphoxide (DMSO) or an isomer ofthe latter.

The extraction of isobutene, and optionally of other branched olefins,in particular isoamylenes, can also comprise an etherification of theisobutene by an alcohol, then a distillation. It is also possible tocarry out a hydroisomerization with reactive distillation, forseparating the isobutene from butene (butene-1 being converted tobutene-2 which can be separated from isobutene).

For the extraction of branched olefins (isobutene and/or isoamylenes)upstream of oligomerization, use of one or more known processes forseparation, such as liquid-liquid extractions, etherifications, or otherprocesses such as membrane processes or using selective adsorbantsoptionally in simulated countercurrent, also falls within the scope ofthe invention.

The several technical variants of the process according to theinvention, described for a combined process of steam cracking andcatalytic cracking, can also be applied when the installation forimplementing the process according to the invention is combined with FCC(refining site), or on a mixed site (steam cracker+FCC), or on anisolated site.

The description of FIG. 1 will now be continued: The olefinic fraction,free from isobutene, and typically comprising paraffins, is recycled tothe oligomerization reactor via line 10. Preferably it is recycled tothe selective hydrogenation reactor R1, via line 15 then line 13 asshown by the dashed line in FIG. 1, before being recycled to theoligomerization reactor R2.

After extracting the isobutene, it is also possible to carry outextraction of paraffins so that a cut without paraffins and withoutisobutene, essentially comprising linear olefins, is recycled or fed tothe selective hydrogenation or oligomerization.

All or preferably a proportion of the gasoline cut obtained fromcatalytic cracking in Stage d) can also optionally be recycled tocatalytic cracking, as is indicated by line 14, shown as a dashed line.The effluents from the catalytic cracking unit other than the recycledC4/C5 cut are evacuated via line 12 as well as via other lines which arenot shown. A proportion or the whole of the C4/C5 cut contained in theeffluents from cracking can also be evacuated, and not recycled.

Very generally, the charge subjected to oligomerization can comprise aC5 olefinic cut, alone or mixed with other olefinic cuts such as C4and/or C2. It was found that better cracking results (propylene yield)were obtained by cracking oligomers formed from a C5 cut, alone orco-oligomerized with other olefins, than the non-oligomerized C5 charge.The improvement relates both to the selectivity of cracking to propyleneand the single-pass conversion of the oligomers, which is increased. Itis thought that this observed increase in single-pass conversion, foroligomers relative to the starting olefins, comes in particular from theincrease in molecular weight, and is notable for all the olefiniccharges envisaged in the present application.

According to another variant, the C4/C5 cut can be recycled withoutextraction of the isobutene. The raw C4/C5 charge, after selectivehydrogenation, is then oligomerized in R2 and separated in S2. S2 canthen comprise only a separation of the oligomers (by distillation), sentto reactor R3 via line 8, with the residual C4/C5 cut (contained in theeffluents from oligomerization), essentially paraffinic, being evacuatedvia line 7 a. It is then preferable to provide evacuation (withoutrecycling) of a proportion of the C4 cut obtained from cracking, and/ornot feed a proportion of the oligomers to the cracking stage, in orderto carry out a direct or indirect purge of isobutene. This evacuation ofoligomers, in particular of oligomers comprising di-isobutene and/ortri-isobutene, can also be carried out within the scope of anoligomerization in one or two stages, as has already been explained.

C6 olefinic fractions can also be recycled.

FIG. 2 describes an installation for implementing a variant of theprocess according to the invention using one and the same catalystcirculating in a fluidized bed in and between three separate zones: theoligomerization reactor R2, the cracking reactor R3, and a zone Z ofcommon regeneration-of the catalyst. The flows of solid are shown bysolid lines and the flows of charge, effluents or various recycles areshown by dashed lines.

The catalyst is regenerated in zone Z at elevated temperatures of theorder of 500° C. to 750° C., pressure levels typically comprised between0.1 MPa and 4 MPa and preferably comprised between 0.1 and 3 MPa, bymeans of a combustion gas, which is generally air or air diluted withnitrogen and/or recycled combustion gas.

The air (or diluted air) is fed to zone Z via line 21 and the combustionfumes are evacuated via line 22.

In zone Z, catalyst regeneration can be carried out in one or morestages at different temperatures and oxygen partial pressures. It isthus possible to burn the coke deposited on the catalyst first with alow oxygen partial pressure, then at higher temperatures and oxygenpartial pressure, without risk of the combustion reaction becomingviolent, by splitting the regeneration process into two stages. Sincezone Z is under an oxygen atmosphere, it is desirable to have bufferzones upstream and downstream of the said zone with inert flows, forexample of nitrogen, so as to prevent any leaks of oxygen via the linesfor transferring the catalyst between reactors R2, R3 and zone Z. Moregenerally, the installation can comprise chemical engineering meanswhich are not shown, such as catalyst hoppers, means for mixing a flowof catalyst with a liquid and/or a gas, chambers for mixing,pressurization or depressurization, wetting, or rendering inert, meansfor stripping, heating or cooling of the catalyst, for example astripper of fluidized solid or a heat exchanger for heating or coolingfluidized solid etc.

A flow of regenerated catalyst is typically sent from zone Z to theoligomerization reactor R2 via transfer line A1, and returns partiallydeactivated from R2 to zone Z via line A2.

Similarly, another flow of regenerated catalyst is typically sent fromzone Z to'the catalytic cracking reactor R3 via transfer line A3, andreturns partially deactivated from R3 to zone Z via line A4.

The installation can also operate in a different mode: The whole of theregenerated catalyst can be sent to reactor R3 via line A3, thentransferred from reactor R3 to reactor R2 via line A6, then returned tozone Z via line A2.

Similarly, the whole of the regenerated catalyst can be sent to reactorR2 via line A1, then transferred from reactor R2 to reactor R3 via linea5, then returned to zone Z via line A4.

The installation can also operate by combining these different modes ofcirculation which then use partial flows of catalyst (and not the wholeof the flow of regenerated catalyst).

Attainment of lower temperatures in oligomerization reactor R2 (comparedwith cracking reactor R3) can be achieved by cooling (or coolingfurther) the flow of catalyst feeding R2 and/or by feeding R2 withreactants at relatively lower temperature. These cooling means are notshown in the figure.

The operating pressures can be similar or practically identical in thethree zones: reactor R2, reactor R3, and regeneration Z to facilitatecirculation of the catalyst, but it is also possible to operate atdifferent pressures, for example at a higher pressure foroligomerization.

The other referenced elements in FIG. 2 have already been described forFIG. 1.

FIG. 2 can also represent an installation for implementing the processaccording to the invention with R2 and R3 operating with a moving bed,not a fluidized bed (with continuous or semi-continuous (intermittent)circulation of a catalyst in the form of particles, for example sphereswith diameter comprised between 1 and 3 mm).

The process according to the invention is not limited to the elementsdescribed above, and, can be implemented according to variants or withembodiments not described in the present description but already wellknown to a person skilled in the art.

EXAMPLE 1 According to the Invention

The following example of application in a pilot plant illustrates theinvention without limiting its scope.

A C4 cut from steam cracking, to which a C4/C5 cut recycled from thecracking stage has been added, is used as the feed for oligomerization.Said cut undergoes selective hydrogenation beforehand and removal of theisobutene that it contained initially. The compositions (as percentagesby weight) of the charge and of the effluent from oligomerization areshown in Table 1.

The operating conditions of the oligomerization zone are as follows:pressure 5.5 MPa, temperature 220° C., SV=1 h⁻¹. The catalyst used is atype MFI zeolite with an Si/Al ratio of 48. It is used in the form ofspheres with average diameter of 2 mm. The oligomers produced, whichmainly contain C8 olefinic oligomers, and C12 in smaller quantities, arefed to the cracking zone, which has the following operating conditions:

-   -   pressure: 0.2 MPa,    -   temperature: 520° C.,    -   SV=10 h⁻¹.

The catalyst used contains 30 wt. % of zeolite ZSM-5, dispersed in asilica-alumina matrix. It is used in the form of spheres with averagediameter of 2 mm.

The cracking yields of the oligomers are shown in Table 2.

The C4/C5 fraction from cracking (apart from isobutene) is thenrecycled, as already mentioned.

Calculation of the overall propylene yield, taking into account the cokebalance (approximately 2% in the cracking stage), gives a yield of 48%relative to the olefins in the initial charge. It might be possible toincrease this yield slightly by recycling the unconverted C4/C5 olefinsleaving oligomerization.

As a variant, if it is desirable to carry out combined production ofpropylene and gasoline (and optionally of kerosene), it is possible toincrease the flow rate in the oligomerization section (without alteringthe cracking) and extract an additional cut of oligomers constituting abase of gasoline and optionally of kerosene.

TABLE 1 wt. % Oligo inlet Oligo outlet C4/C5 paraffins 11.99 15.65Isobutene 0.0 0.0 C4/C5 olefins (except isobutene) 88.01 0.83 Oligomers0.0 83.52 Total 100.0 100.0

TABLE 2 wt. % Cracking outlet H2 0.41 CH4 1.22 C2H4 4.59 C2H6 2.24 C3H640.79 C3H8 3.57 C4/C5 paraffins 6.01 Isobutene 11.83 C4/C5 olefinsexcept isobutene 25.26 Gasoline 4.08 Total 100.0

EXAMPLE 2 Oligomerization of Various Olefinic Charges

Two C4 and C5 olefinic cuts are available (which may for example comefrom an FCC), for which a preliminary oligomerization is carried out:either of the C4 cut alone, or of the C4 cut mixed with the C5 cut(co-oligomerization). The effluents from these oligomerizations will beused in Examples 3 and 4 below.

The conditions of oligomerization are identical to those in Example 1.

The composition of the charges and of the effluents from oligomerizationis shown in Table 3:

TABLE 3 Charge or Effluent Effluent from from Effluent fromoligomerization oligomerization C4 cut C5 cut oligomerization of the(C4 + kg/h (charge) (charge) of the C4 cut C5) cut C4 paraffins 7649 07871 7871 Isobutene 125 0 6 6 Butene-1 2025 0 103 106 Butene-2 5069 0254 324 C5 paraffins 0 7598 0 7844 C5 olefins 0 9838 0 2103 C6⁺ cut (1)0 0 6633 14048 Total 14867 17435 14867 32302 (1) A Cn⁺cut comprises, bydefinition, the cuts having at least n carbon atoms.

EXAMPLE 3 According to the Invention

The C5⁺ cut obtained from oligomerization (co-oligomerization) of the(C4+C5) cut according to Example 2 (last column of Table 3) is crackedcatalytically in the same conditions as in Example 1. The crackingyields, expressed as in the case in Example 1, as wt. % relative to theolefinic cut of the C4+C5 charge, except isobutene, are shown in Table4:

TABLE 4 Yields, wt. % Cracking outlet H₂ 0.39 CH4 1.15 C2H4 5.20 C2H62.44 C3H6 28.73 C3H8 3.35 C4/C5 paraffins 5.65 Isobutene 11.11 C4/C5olefins except isobutene 37.40 Gasoline 4.60 Total 100.0

If the cracking reactor, for example in an FCC unit or a fixed-bed ormoving-bed unit, or the cracked gas compressor, or the gas treatmentunit are of limited capacity, it is possible to use the C6⁺ cut obtainedfrom oligomerization, rather than the C5⁺ cut obtained fromoligomerization, as the feed for cracking, to reduce the flow rate ofcracked oligomers, with a relatively small loss of propylene.

If the charge of oligomers sent to cracking is to be reduced further,only the C8⁺ or even C9⁺ fraction need be sent for cracking. Thesedifferent variants can also be used in the case when oligomerization iscarried out in 2 Stages b1) and b3). It is possible for example to carryout a first oligomerization b1) of a C4/C5 cut, evacuate the C8⁺oligomers to a fractionation zone for preparation of bases of gasolineand/or of kerosene, feed the residual C4/C5 fraction to a secondoligomerization b3), then separate the second C8⁺ or C9⁺ oligomers whichare used as feed for cracking.

EXAMPLE 4 Comparative

The initial C5 cut (i.e. without prior oligomerization), to which theC5⁺ fraction contained in the effluents from oligomerization of the C4cut according to Example 2 has been added, is cracked catalytically inthe conditions of Example 1. In this comparative example only the C4 cutis oligomerized (typically for the purposes of removing the butanesafter this oligomerization and before cracking). The C5 cut, which has arelatively lower content of paraffins than the C4 cut, is crackeddirectly, mixed with the C5⁺ oligomers obtained by oligomerization ofthe C4 cut. The cracking yields are given in the following table:

TABLE 5 Yields, kg/h Cracking outlet H2 0.24 CH4 0.73 C2H4 4.87 C2H62.13 C3H6 18.17 C3H8 2.12 C4/C5 paraffins 3.57 Isobutene 7.03 C4/C5olefins except isobutene 59.03 Gasoline 2.11 Total 100.00

It can be seen that the propylene yield in this comparative example isfar lower than that of Example 3 according to the invention. This showsthe benefit of co-oligomerizing complex cuts comprising olefins havingdifferent numbers of carbon atoms, before cracking is carried out,namely for improving the selectivity for propylene and the single-passconversion (Note: Examples 3 and 4 cannot be compared with Example 1because, as well as the different charge, they correspond to single-passcracking, without recycling of the olefins that re-formed duringcracking).

The process according to the invention therefore makes it possible,according to different variants, to improve the production of propylenefor a given charge. It also makes it possible to reduce recycling asthere is higher conversion per pass, and typically to improve theselectivity of cracking towards propylene. It can also permit theco-production of gasoline with a high octane number (in particular ofgasoline rich in di-isobutene) and of propylene, or of di-isobutene(mixture of the two isomers) and of propylene, or of bases of gasolineand/or of kerosene and/or of gasoil, and of propylene.

1. A process for the catalytic conversion of a hydrocarbon chargecomprising olefins, said process comprising the following sequence ofstages: at least one Stage b) or b1) or b3) of oligomerization in whicha catalytic oligomerization of said olefins is carried out in at leastone reactor to obtain oligomers, said hydrocarbon charge comprising byweight at least 70% of C4+C5+C6 olefins, at least 10% of C₄ olefins andat least 10% of C₅ olefins, with a ratio R₂ of C₅ olefins to C₄ olefinshigher than 0.5 and less than 2, and a ratio of R₃ of(C₄+C₅ olefins) toC₆ olefins of greater than 10, a Stage d) catalytic cracking of aoligomerizate charge containing a proportion of at least said oligomersis carried out in a reactor different from the oligomerization reactor,to produce propylene.
 2. A process for the catalytic conversion of ahydrocarbon charge comprising olefins according to claim 1, comprisingisobutene, said process comprising the following sequence of stages: atleast one Stage b) or b1) or b3) of oligomerization in which a catalyticoligomerization of the olefins contained in the charge is carried out inat least one reactor, then, a Stage b2) or c) of fractionation in whicha proportion at least of the oligomers produced is separated, and isevacuated directly without feeding the subsequent Stage d), saidevacuated proportion comprising di-isobutene and/or tri-isobutene, aStage d) in which catalytic cracking of a proportion at least of theoligomers produced is carried out in a reactor different from theoligomerization reactor, to produce propylene.
 3. A process according toclaim 2, in which the following are carried out upstream of Stage d): afirst stage of limited oligomerization b1), a Stage b2) of fractionationof the effluents of Stage b1), to produce at least one cut which isevacuated directly without feeding to subsequent Stages b3), d), saidevacuated cut comprising di-isobutene and/or tri-isobutene, a Stage b3)of final oligomerization of the effluent of Stage b2) or at least of C4and/or C5 olefinic fractions contained in said effluent, afterevacuation of the aforementioned cut comprising di-isobutene and/ortri-isobutene, and cracking the resultant final oligomerizate in Stage(d).
 4. A process according to claim 3, in which the following arecarried out: Stage b1) with a charge essentially consisting of a C4 cutalone, Stage b3), adding a C5 or C2+C5 cut to the butenes that were notconverted in b1).
 5. A process according to claim 1, in which thehydrocarbon charge to the oligomerization reactor comprises from 0.5 to15 wt. % of ethylene.
 6. A process according to claim 1, in which thehydrocarbon charge to the oligomerization reactor comprises from 0.5 to15 wt. % of ethylene relative to the total of the C4,C5 and C6 olefins.7. A process according to claim 1 ,in which the hydrocarbon charge tothe oligomerization reactor comprises diolefinic and/or acetyleniccompounds, and in which said hydrocarbon charge is first subjected to aStage a) of selective hydrogenation for practically eliminating saiddiolefinic and/or acetylenic compounds.
 8. A process according to claim1, in which the catalyst used for the oligomerization stage comprises anacidic solid possessing shape selectivity, said catalyst comprising atleast one zeolite, said zeolite comprising silicon and at least oneelement chosen from the group formed by aluminium, iron, gallium,phosphorus, boron, and aluminium, the zeolite exhibiting shapeselectivity used being from the group comprising the zeolites of one ofthe following structural types: MEL, MFI, NES, EUO, FER, CHA, MFS, MWW,or from the group of the following zeolites: NU-85,NU-86,NU-88 and IM-5.9. A process according to claim 1, in which the catalyst of Stage d) ofcatalytic cracking comprises a zeolite, cracking being carried out at atemperature comprised between 450° C. and 650° C., and a pressurecomprised between 0.1 and 0.5 MPa.
 10. A process according to claim 1,in which the catalyst used for the cracking stage comprises a zeoliteexhibiting shape selectivity of structural type MFI, alone or mixed withanother zeolite exhibiting shape selectivity chosen from the groupcomprising the zeolites of one of the following structural types: MEL(for example ZSM-11), NES, EUO, FER, CHA (for example SAPO-34), MFS,MWW, or the group of the following zeolites: NU-85,NU-86, NU-88 andIM-5,and in which the zeolite or zeolites used exhibiting shapeselectivity have an Si/Al ratio greater than
 12. 11. A process accordingto claim 1, in which the catalyst used for the cracking stage comprisesone or more zeolites exhibiting shape selectivity, the proportion ofzeolite(s) exhibiting shape selectivity being comprised between 70 and100 wt. % relative to the total quantity of zeolite(s).
 12. A processaccording to claim 8, in which the catalyst used in the cracking stagecomprises a zeolite exhibiting shape selectivity with Si/Al ratiodifferent from the zeolite or zeolites exhibiting shape selectivitycontained in the catalyst used in the oligomerization stage.
 13. Aprocess according to claim 1, in which the catalytic cracking reactor isa fixed-bed or moving-bed or fluidized-bed reactor.
 14. A processaccording to claim 1, wherein the ratio R₂ is greater than 0.7 and lessthan 1.5.
 15. A process according to claim 1, wherein the oligomerizatecharge to the cracking stage contains 20-100% by weight of olefins withat least 8 carbon atoms, and includes C9 co-dimers.
 16. A processaccording to claim 1, wherein the oligomerizate charge to the crackingstage contains 30-100% by weight of olefins with at least 8 carbonatoms, and includes C9 co-dimers.
 17. A process according to claim 1,wherein the oligomerizate charge to the cracking stage contains 50-100%by weight of olefins with at least 8 carbon atoms, and includes C9co-dimers.
 18. A process according to claim 1, wherein the oligomerizatecharge to the cracking stage contains 60-100% by weight of olefins withat least 8 carbon atoms, and includes C9 co-dimers.